1. Field of the Invention
The present invention relates to a method of reforming a capacitor in an implantable medical device, such as an implantable cardioverter defibrillator (ICD). The present invention also relates to a capacitor reformed by the method of the invention and an implantable medical device incorporating a capacitor reformed by the method of the invention. The present invention further relates to an implantable medical device having reforming circuitry for reforming the capacitor in the implantable medical device.
2. Background Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. Implantable medical devices include implantable cardiac devices such as, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device or implantable cardioverter defibrillator (“ICD”).
ICDs are typically implanted in patients suffering from potentially lethal cardiac arrhythmias. Arrhythmia, meaning “without rhythm,” denotes any variance from normal cardiac rhythm. Heartbeat irregularities are fairly common and many are harmless. A severe heartbeat irregularity known as ventricular tachycardia refers to a runaway heartbeat.
Fibrillation is an irregular rhythm of the heart caused by continuous, rapid, electrical impulses being emitted/discharged at multiple locations known as foci in the heart's atria and ventricles. Because a fibrillating heart is unable to properly pump blood through a patient's body, the longer a patient is in fibrillation, the greater the potential damage that can occur to the patient's heart. Thus, after the start of fibrillation, it is preferable to apply defibrillating therapy to the patient as soon as possible. An ICD is designed to apply such therapy automatically and quickly to minimize damage to the heart.
An ICD monitors cardiac activity and decides whether electrical therapy is required. For example, if a tachycardia is detected, pacing or cardioversion therapy may be used to terminate the arrhythmia. If fibrillation is detected, defibrillation is the only effective therapy.
Typical ICDs include a set of electrical leads, which extend from a sealed housing into the walls of a heart after implantation. Within the housing are a battery for supplying power, a capacitor for delivering bursts of electric current through the leads to the heart, and monitoring circuitry for monitoring the heart and determining when, where, and what electrical therapy to apply. The monitoring circuitry generally includes a microprocessor and a memory that stores instructions not only dictating how the microprocessor controls delivery of therapy, but also controlling certain device maintenance functions, such as maintenance of the capacitors in the device.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Typically, these capacitors can be aluminum electrolytic capacitors having aluminum foil plates. Current ICDs usually contain two aluminum electrolytic capacitors for shock delivery.
It is important that the anode foil used in these capacitors maintains a high capacitance with the lowest possible leakage current. The term “leakage current” refers to the current passing from the cathode plate through an electrolyte and across the anodic oxide dielectric into the aluminum foil. Under conventional anode foil preparation techniques, a barrier oxide layer is formed onto one or both surfaces of a metal foil. The oxide film must be sufficiently thick to support the intended use voltage for shock delivery (referred to hereinafter as the “nominal voltage”). This oxide film acts as a dielectric layer for the capacitor, and constitutes a barrier to the flow of current between the electrolyte and the metal foil, thereby providing a high resistance to leakage current passing between the anode and cathode foils. However, a small amount of current, the leakage current, still passes through the barrier oxide layer due to intrinsic defects in the crystalline oxide, and electron injection from the electrolyte rather than oxide injection into the dielectric. A high leakage current can result in the poor performance and reliability of an electrolytic capacitor. In particular, a high leakage current results in a greater amount of charge lost internally to the capacitor once it has been charged.
Both cardioversion and defibrillation require that a high voltage shock be delivered to the heart. Since ICDs are typically powered by a battery implanted in a patient's body, it is usually impractical to maintain full voltage continuously ready for use. To conserve battery energy, ICDs normally charge energy storage capacitors after detection of an arrhythmia and prior to delivering a shock to the heart.
To shorten the time between arrhythmia onset and therapy, pulse discharge capacitors such as those in ICDs are required to charge quickly after protracted storage in the discharged state. However, leaving the capacitors in an uncharged state leads to degradation of the aluminum oxide on the capacitors over time. Instability of the aluminum oxide in the liquid electrolyte results in degradation over time of the charging efficiency of the capacitor. For this reason, ICDs containing aluminum electrolytic capacitors typically also include capacitor maintenance software to periodically reform the aluminum oxide on the aluminum electrolytic capacitors. The periodic reformation process serves to replenish the oxide and reduce the leakage current of the aluminum electrolytic capacitors. This, in turn, reduces charge time of the capacitors the first time that they are needed for therapeutic use after an extended period of non-use.
Conventionally, the reformation process consists of charging the aluminum electrolytic capacitors to the device's nominal voltage and then allowing the charge to dissipate.
A Capacitor Maintenance Interval is generally established with a range of 1-6 months. When the Capacitor Maintenance Interval times out, the device performs Capacitor Maintenance. Typically, Capacitor Maintenance consists of the ICD's software requesting charging of the capacitors to the device's nominal voltage. After the Capacitor Maintenance charge to the device's nominal voltage is completed, the Capacitor Maintenance Interval is restarted. The charge on the capacitors is allowed to dissipate by leaking through some parasitic discharge path. Alternatively, the ICDs may be programmed to dump the capacitor charge into an internal load.
Various other systems for capacitor maintenance exist, including automatic capacitor maintenance systems such as those described in U.S. Pat. No. 5,861,006 to Kroll, and U.S. Pat. No. 5,899,923 to Kroll et al., which are incorporated herein by reference in their entirety.